linux-kernel - Re: [PATCH 3/4] KVM: s390: replace bitmap_copy with bitmap_{from,to}

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Date:   Thu, 21 Apr 2022 06:01:23 -0700
From:   Yury Norov <yury.norov@...il.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Sven Schnelle <svens@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>, linux-s390@...r.kernel.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH 3/4] KVM: s390: replace bitmap_copy with
 bitmap_{from,to}_arr64 where appropriate

On Thu, Apr 21, 2022 at 09:24:20AM +0200, David Hildenbrand wrote:
> On 21.04.22 00:25, Yury Norov wrote:
> > Copying bitmaps from/to 64-bit arrays with bitmap_copy is not safe
> > in general case. Use designated functions instead.
> > 
> 
> Just so I understand correctly: there is no BUG, it's just cleaner to do
> it that way, correct?

Yes. there's no bug, but the pattern is considered bad.

https://lore.kernel.org/all/YiCWNdWd+AsLbDkp@smile.fi.intel.com/T/#m9080cbb8a8235d7d4b7e38292cee8e4903f9afe4q
 
> IIUC, bitmap_to_arr64() translates to bitmap_copy_clear_tail() on s390x.

Yes.
 
> As the passed length is always 1024 (KVM_S390_VM_CPU_FEAT_NR_BITS), we
> essentially end up with bitmap_copy() again.
> 
> 
> Looks cleaner to me

Thanks.

> Reviewed-by: David Hildenbrand <david@...hat.com>
> 
> 
> -- 
> Thanks,
> 
> David / dhildenb